a) Field of the Invention
This invention relates to a process for the fabrication of a wiring board.
b) Description of the Related Art
The etched foil process has been used most widely as a process for the fabrication of wiring boards. According to the etched foil process, a wiring board is fabricated by forming an etching resist on a copper clad laminate, etching off the copper foil with an etchant at parts, where the etching resist is not formed, to form a desired wiring pattern, and then removing the etching resist.
Although the etched foil process is simple and is excellent in productivity, it cannot form a high-density or high-accuracy pattern because the thickness of the copper foil of a copper clad laminate employed in the process is generally as thick as 35-70 .mu.m so that the side walls of the conductive lines of the resulting wiring pattern are undercut in a step in which the copper foil is etched off at parts where no etching resist is formed.
Further, the conductive lines of the wiring pattern protrude from the surface of the wiring board so fabricated. When another wiring pattern is formed over the conductive lines of the first-mentioned wiring pattern through an insulating layer to fabricate a multilayer wiring board, the rugged profile of the surface of the first-mentioned wiring board poses a problem for the fabrication of a high-density multilayer wiring board.
A process has been proposed, in Japanese Patent Publication No. SHO 54-35670, for example, to fabricate a wiring board in which wiring conductive lines are embedded in a substrate to present a smooth surface.
According to the process disclosed in the above patent publication, wiring conductive lines are formed on a stainless steel sheet, the stainless steel sheet is pressed under heat against an insulating substrate with a prepreg interposed therebetween, and the stainless steel sheet is then removed. Since the wiring conductive lines are directly formed on the stainless steel sheet, some damage may be caused on the wiring conductive lines upon removal of the stainless steel sheet so that a limitation is imposed on the formation of a high-density and high-accuracy wiring pattern. In addition, the use of the stainless steel sheet does not allow to perform the fabrication in a continuous flow.
In the meantime, a wide variety of investigations have been conducted regarding the fabrication of a multilayer wiring board to achieve high densification of a wiring board. Conventional fabrication processes, namely, interconnection process proposed so far for the production of a multilayer wiring board include:
(1) After forming holes through a wiring pattern and an insulating layer, the insides of the holes are metalized by plating or the like so as to achieve interlayer conduction. PA1 (2) Subsequent to formation of an insulating layer on a wiring pattern, the insulating layer is removed at locations where interconnection is desired, followed by surface metallization and simultaneous interconnection.
The process (1) is an interconnection process which is practiced upon fabrication of general multilayer wiring boards. This process however uses a drill for the formation of through-holes. It is therefore difficult to form holes of a diameter smaller than 0.2 mm. Moreover, the positional accuracy of such holes is .+-.30 .mu.m minimum and is hence insufficient.
The process (2), on the other hand, is used primarily for the formation of a multilayer wiring in semiconductor devices. Owing to the use of a positioning technique which makes use of a photomask, the process (2) is superior by one figure to the process (1) in the diameter reduction of interconnected parts and the positional accuracy of holes. The process (2), however, cannot absorb the ruggedness of an underlying wiring pattern so that the surface remains rugged. This has posed an obstacle not only for the formation of additional layers or the mounting of devices on its surface but also the formation of the surface wiring into a microstructure.
Interconnecting holes are formed with a small diameter in both the process (1) and the process (2) so that a plating solution or the like is not allowed to flow smoothly into the holes. This makes it difficult to form a metal film of a sufficient thickness. This is a serious problem because it directly affects the reliability of the resulting interconnection.